1. Field of Invention
This invention relates to a sensor used,for example, to sense a process fluid; and more particularly, to a sensor housing used therein.
2. Description of the Prior Art
A conventional sensor housing is disclosed, for example, in U.S. Pat. No. 4,128,468 which describes an ion sensitive electrode structure which can be used, for example, to measure the pH of a process fluid. This structure comprises an elongated tubular housing which holds a glass electrode and a reference electrode therein. The elongated housing is sealed toward one end by means of a sealing annulus through which signal wires of the electrodes are passed. The other end of the housing is brought into contact with the process fluid to be measured. This end of the housing is sealed by a plug of porous material, for example, porous polytetrafluoroethene (PTFE). Through the porous plug, the reference electrode is in electrochemical contact with the process fluid. Located in the plug is a passage into which the glass electrode is disposed, with the result that the sensitive end of the glass electrode is projected outwards at the process end of the housing.
The process industry generally makes use of a thickened electrolyte in the pH sensor to prevent unduly rapid loss of the electrolyte, particularly when a relatively highly porous material is used for the plug which serves as the liquid junction. Because of the relative size (about 10 microns) of the porous PTFE material of the plug, unduly rapid loss of electrolyte can occur if the electrolyte is not sufficiently viscous, thereby significantly limiting the service life of the pH sensor. The thickened electrolyte has a drawback, however, namely, that its volume increases relatively markedly with increase of temperature. The volume can increase by the order of magnitude of from 5% to 10%. If the walls of the housing which hold the thickened electrolyte are stiff, and if the temperature coefficient of the material of the housing is such that the increase in volume of the housing is less than that of the electrolyte, considerable pressure will build up in the sensor. This may lead to rupture of the housing, to the plug being pushed away, or to loss of electrolyte. In the first two cases, electrolyte would disappear from the sensor and the sensor will become useless. In the third case, a decrease in temperature will result in the ingress of the process fluid to make for loss of electrolyte, thereby contaminating the electrolyte. This would lead to uncalibrated operation of the pH sensor and Consequent unreliable reading.
U.S. Pat. No. 4,659,451 discloses a reference electrode for measuring fluid under high pressure. The reference electrode can form part of a pH measuring circuit and is provided with an internal liquid containing space therein and a liquid junction, one end of which is exposed to the process fluid and the other end of which is exposed to the liquid contained in the liquid containing space. Flexible bellows are provided between the spaces in contact with the internal liquid containing body to compensate for the pressure, of the high pressure liquid, which is exerted on the internal liquid through the liquid junction. All of these components are acccommodated in a sensor housing which is rigid.
The flexible bellows are designed to transmit the pressure of the process fluid to the inside of the reference electrode. To this end, the pressure of the process fluid needs to be transmitted through a capillary to the flexible bellows. The capillary can easily become clogged by the process fluid resulting in the pressure being partially or entirely blocked from transmission. Moreover, in this sensor, various junctions are present where various parts of the sensor are fastened to one another. For example, the flexible bellows are fastened to the glass reference electrode by fastening means. However, disadvantageously, the fasteners inherently form the weak point of the sensor.
U.S. Pat. No. 4,406,766 discloses a pH sensor for measuring the pH of a process fluid under elevated temperature and pressure, wherein the sensitive parts of the measuring electrodes are not exposed to the high temperatures and where no contamination of internal electrolyte occurs upon pressure changes in the process fluid. This is achieved by the sensitive parts of the measuring sensors being arranged at a distance from the end of the pH sensor which comes into contact with the process fluid. Attachment of the pH sensor to a vessel or like containing process fluid, is effected by means of a number of thermal barriers, so that the sensitive parts of the pH sensor remain at roughly ambient temperature. Attached to the ends of the glass electrode and reference electrode, which are at a distance from the end of the pH sensor in contact with the process fluid, are deformable wall bodies contained in a chamber. The pressure of the process fluid can be transmitted through a capillary to the chamber. The pressure of the process fluid is thereby transmitted at the same time through the deformable wall bodies to the electrolyte present at the glass electrode and reference electrode. Hence, the internal pressure of the glass electrode and the reference electrode is equal to the pressure in the process fluid.
However, this known pH sensor has a number of problems. First, the deformable wall bodies are made of a material which is different from that of the glass electrode and the reference electrode, which means that the junction is inherently weak. Also, the pH sensor is provided with a capillary for transmitting the pressure. This capillary can easily become clogged, thereby resulting in partial or complete loss of pressure compensation, which in turn results in fracture or leakage of the electrolyte.
Thus, the conventional sensors all have various disadvantages and deficiencies.
Accordingly, an object of the invention is to overcome the aforementioned and other deficiencies and disadvantages of the prior art.
Another object is to provide a sensor housing, such as for example for use in a pH sensor, which is temperature resistant and pressure proof. That is to say, with the invention, over an entire specified pressure and temperature range of the sensor, no process fluid will be able to penetrate the sensor even when the pressure and temperature variations would otherwise cause contamination of the electrolyte.
The foregoing and other objects are attained by the invention, which in one aspect comprises a sensor housing for accommodating at least one electrode which contacts a process fluid through a porous opening of an inner chamber formed between an inner surface of the sensor housing and an external surface of the at least one electrode with the inner chamber being filled with electrolyte, wherein one or more deformable sections are provided to enable the space of the inner chamber to be variable, that is to allow the space to increase or decrease in volume.
An advantage of the invention sensor using the invention housing is that any expansion, caused by a higher temperature of the process fluid, or of the thickened electrolyte is absorbed by the one or more deformable sections of the sensor housing. As a result of out-ward deformation of the at least one deformable section, the internal volume of the space of the sensor increases, and no pressure build up within the housing occurs. Thus, no leakage of electrolyte will take place upon subsequent cooling of the sensor with the otherwise possible contamination of the electrolyte.
In a preferred embodiment, the sensor housing and deformable sections are made of one material. This allows for simple fabrication of the sensor housing. Also, there is absence of junctions between different materials which prevents occurrence of fractures and leaks in the housing.
In a further embodiment, the material of the sensor housing, with the exception of the deformable sections, has a second material added thereto. The second material can be a stiffening material which will provide a stiffer and more robust sensor housing, while the deformable sections remain sufficiently flexible to provide a variable internal space.
In a further embodiment, the housing and deformable sections are made of one piece construction. Thus, there is no junction between parts of the sensor housing and hence the sensor housing has no inherent weakness and is more resistant to external influences.
In a still further embodiment, the at least one deformable section or sections are located on that part of the housing which is contactable with the process fluid. This allows the pressure of the process fluid to be transmitted through the deformable sections to the internal space of the housing. As a result, no pressure differentials will occur between the inner chamber and the process fluid. This avoids use of a capillary means, hence, the invention is not vulnerable to blockage.
Preferably, in another embodiment of the sensor housing, the housing is provided with one or more bracing ribs at the locations of the deformable sections. As a result, the sensor housing is more rigid, which means that the housing can be attached more readily and with no risk of damage to the interior thereof, for example, in a process vessel. Furthermore, this also results in better protection of the electrodes placed within the housing, for example, a glass electrode and a reference electrode which generally comprises rigid glass parts.
In another embodiment, the material of the sensor housing is resistant to chemicals. For example, the material may be a plastic, such as PVDF, which offers good resistance up to pH values of 12 and above, depending on the temperature.
In a further embodiment, the one or more deformable sections are formed by local attenuations of walls of the sensor housing. Preferably, the deformable sections have a maximum thickness of 0.50 mm, for example, a thickness of 0.25 mm. This provides sufficient deformability of the sensor housing to absorb variations in the space of the inner chamber. Advantageously, it is possible to use standard fabrication techniques to fabricate the sensor housing of the above discussed thicknesses.
A second aspect of the invention encompasses a sensor comprising a sensor housing of the invention. One embodiment relates to a pH sensor comprising a reference electrode which is located in the inner chamber of the sensor housing.
In a further embodiment, the sensor further comprises a glass electrode to measure the pH of a process fluid. A porous seal is provide with a passage into which the glass electrode is fitted.
A yet further embodiment relates to a sensor which is additionally provided with a temperature sensor disposed in the sensor housing.
A still further embodiment of the sensor is an additionally provided ORP electrode or liquid earth electrode which is disposed in the sensor housing and having a contact projected through the porous seal to contact the process fluid. The liquid earth electrode can also be used for diagnostic purposes relating to action of the sensor during operation.
A third aspect of the invention relates to a method of fabricating the sensor housing of the invention. For example, the sensor housing can be fabricated using an injection moulding process or alternatively a vacuum forming process.
The one or more deformable sections of the sensor housing can be fabricated integrally, or alternatively, the one or more deformable sections can be fabricated by partial removal of the material thereof. An alternative fabrication technique is to apply the one or more deformable in a further procedural step using welding process. As a result, a sensor housing is obtained of one piece construction with a section being deformable so that an inner chamber having a space or volume which is variable, that is space that can be increased or decreased, is obtained. The one or more deformable sections can be obtained also by use of a glued joint.